This invention relates to the non-invasive measurement of the concentration of substances that absorb electromagnetic radiation, such as light or infrared radiation, in absorbing and turbid matrices. In particular, the invention is directed to substances, such as glucose, found in the blood of absorbing and turbid matrices, such as human or animal body tissue.
Numerous techniques have been proposed for the determination of glucose by non-invasive optical monitoring methods. (See "Blood Glucose Sensors: An Overview", by Peura, R. A. and Mendelson, Y., Proceedings of the IEEE/NSF Symposium on Biosensors, Los Angeles, Calif., 1984.) Many of the proposed methods rely on transmissive and diffuse reflective absorption measurements using infrared radiation.
The infrared measurement methods known in the art are not well adapted to the problem of quantifying an analyte dissolved in a strongly absorbing solvent. The known methods include separate or directly alternating measurements using radiations at a "glucose" wavelength and at a "reference" wavelength, where glucose does not absorb, as well as differential wavelength modulation about a glucose absorption band (C. Dahne, D. Gross, European Patent 0 160 768 and references therein). In the known methods, the signal is easily lost into the strong background presented by the water in tissues and in the capillary blood flow. The normal concentration range of glucose in blood for male adults is 4 to 6 mmol/l (70 to 110 mg/dl).
A need still exists, therefore, for a non-invasive method and apparatus having sufficient long-term sensitivity to accurately measure the concentration of light absorbing substances in absorbing and turbid matrices found in the human or animal body.